Why was the basis of classification of elements changed from the atomic mass to the atomic number?

The basis of classification of elements changed from the atomic mass to atomic number due to

  1. The properties of elements depend upon the number of electrons present in the valence shell which are related to atomic number.
  2. Thus the properties of different elements can be compared if we know their atomic numbers.
  3. On the other hand, atomic mass can in no way determine the chemical proper-ties of elements, because it does not vary regularly with gradation in the chemical properties of elements.